Observed Low Frequency Fluctuations in Space

Abstract

Measurements in the magnetosphere of low frequency magnetic fluctuations less than 1 KHz have been obtained recently with the OGO, ATS and IMP series of satellites. The OGO results indicate the existence of magnetically noisy regions of limited spatial extent within the magnetosphere. Fluctuations between 1Hz-1 kHz show two distinct classes of occurrence, steady and burst occurrences. The general character of the steady noise properties is in agreement with lower altitude ELF hiss measurements. The burst events may be associated with whistler loss cone instabilities. Transverse low frequency fluctuations with periods between 1 to 6 min have been observed on the ATS-1 satellite. Monochromatic oscillations of peak amplitude 2–20 gammas appear to represent the second harmonic of a standing Alfvén wave on a field line. Longer period oscillations of 6 or 8 min have also been observed and may represent an eigenmode oscillation of the magnetopause boundary. Compressional oscillations of the geomagnetic tail with periods of ~ 20 min have been observed by Explorer-33 and probably represent the eigenmodes of the geomagnetic tail. A review of observations and theoretical studies of magnetic fluctuations below 1 kc is given in this paper.

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Ness N.F. (1969) Observed Low Frequency Fluctuations in Space. In: D’angelo N. (eds) Low-Frequency Waves and Irregularities in the Ionosphere. Astrophysics and Space Science Library (A Series of Books on the Recent Developments of Space Science and of General Geophysics and Astrophysics Published in Connection with the Journal Space Science Reviews), vol 14. Springer, Dordrecht